1. Technical Field
The present invention generally relates to searching data relating to a spatial region using a prioritized search order, and more particularly to a Terrain Awareness Warning System (“TAWS”) for use by an aircraft for searching terrain elevation data for a geographic area to determine aircraft terrain clearance.
2. Background Art
Various systems are known in the art that provide warnings and advisory indications of hazardous flight conditions. Among such systems are systems generally known as Ground Proximity Warning Systems (“GPWS”) which monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that inadvertent contact with terrain is imminent. Among flight conditions normally monitored by such systems are radio altitude and rate, barometric altitude and rate, air speed, and flap and gear positions. These parameters are monitored and an advisory signal and/or warning signal is generated when the relationship between the parameters is such that terrain impact is likely to occur.
Other systems improve upon earlier GPWS by utilizing ground-based navigational information. Monitoring stored terrain data and providing modified ground proximity warnings may provide more accurate warnings. U.S. Pat. Nos. 4,646,244, and 4,675,823 each disclose terrain advisory systems that utilize ground-based navigational systems and stored terrain data to provide various ground proximity warnings in relation to the position of the aircraft. U.S. Pat. No. 5,839,080 discloses a Global Positioning System (“GPS”) and stored terrain data to provide warning indications.
Satellite-based navigational systems, such as GPS, which can track longitude, latitude, altitude, groundtrack, and ground speed, are becoming an important and reliable source of information for aircraft. An aircraft's Forward Looking Terrain Avoidance (“FLTA”) system looks ahead of the aircraft during flight along and below the aircraft's lateral and vertical flight path to provide suitable alerts if a potential threat exists of the aircraft colliding or coming too close to terrain. The computation involves searching through a terrain database for terrain cells that are within the search area and violate the Required Terrain Clearance (“RTC”). The RTC is the value set by the Federal Aviation Administration as the permitted flight “floor” for various phases of aircraft flight. The RTC indicates the clearance distance from terrain below which the aircraft should not fly. Searching the search area and finding the cells in violation is expensive in both processor and memory resources.
Two common methods of searching terrain data are sequential and radial. Both of these methods suffer from the deficiency that they expend precious processor and memory resources. For the sequential search method, some common deficiencies include: First, every cell in a rectangular area encompassing the search area is searched, even those cells that are outside the search area boundary. This requires a determination of whether a cell is within the search area or not, which could be complicated and expensive. Second, a large data buffer is needed to store the cells found during the search, the dimension of the buffer needed is difficult to determine accurately because the upper bound could equal the size of the search area which varies by the speed of the plane. Third, sorting the cells in the result buffer by distance to the aircraft position is expensive, especially if a large number of cells are returned in the search and if both distance and bearing are considered.
For the radial search method, some common deficiencies include: First, many possible flight directions are searched, even some of those that are highly unlikely. This requires expensive processing time to be spent on searching all flight directions even when the aircraft is flying straight. Second, because the search is performed in a plurality of radial arms originating at the same point and extending in a fan shape over the underlying data sectioned into a square grid, many cells near the origin of the arms are searched numerous times, and some cells farther from the origin are potentially missed if the radial arms become too far apart at their extents. Third, the position of each point on each radial arm is calculated and terrain data is searched throughout the full database. This is complicated and expensive. Fourth, a large data buffer is needed to store the cell identities found during the search, and the dimension of the buffer needed is difficult to determine accurately because the upper bound could equal the full number of the search points on all radial arms. Fifth, in systems which sort the results, sorting the cells in the result buffer by distance to the aircraft position is expensive, especially if a large number of cells is returned in the search and if both distance and bearing are considered.
Both conventional systems require extensive processor search time and memory storage for crucial Terrain Awareness and Warning systems (“TAWS”), for which the costs of developing, repairing and maintaining are increasing. The additional time required for these searches also reduces the time available for the processor to perform other tasks and lengthens the time in which search results can be made available to a pilot of the aircraft.